Progressive cavity pumps handle a wide variety of materials, from liquids to highly viscous and abrasive materials. One such prior art pump is known as the Moyno progressive cavity pump. This pump is manufactured by Robbins & Myers, Fluids Handling Division, Springfield, Ohio. These pumps can be used to transfer the most abrasive materials. Examples are slurries containing particulate solids such as sand; municipal and industrial waste sludge; slurries of grease, paper pulp, and clay; crude oil, which contains sand, silica or diatomaceous earth; roofing materials; concrete; stucco; and so on.
Progressive cavity pumps generally include a long, narrow hard metal rotor with a single helical recess formed in its outer surface and progressing along the length of the rotor. During operation, the single helix rotor revolves eccentrically within a double helix elastomeric stator of twice of the pitch length. As the rotor turns within the stator, cavities form which progress toward the discharge end of the pump, carrying the fluids with them.
The rotor has for years been made from steel which is plated with a heavy outer layer of dense hard chrome. The stator is an elastomeric sleeve made from specially compounded elastomers and natural rubber. The steel rotor requires an initially smooth surface to avoid abrading the inside of the elastomeric stator. When the pump is used for pumping highly abrasive materials, the elastomeric stator wears out often and requires replacement. The steel alloy from which the rotor is made is not a good abrasion-resistant material, hence, the need for the abrasion-resistant chrome outer layer. Despite use of the hard chrome surface, these rotors wear out often and require replacement when the pump is used for pumping highly abrasive materials.
In the past, the helical rotors used in progressive cavity pumps have been made from steel bar stock (long, straight bars of steel). This material is reasonably inexpensive owing to its availability in large lots from steel mills. The steel bar stock is easily machined, and the rotor is formed by first machining the helical recess into the bar stock on a lathe. Since the machined steel piece is much too soft to have a suitable wear life during use in a cavity pump, the layer of hard chrome is then plated onto its outer surface. The chrome outer layer is commonly plated to a uniform layer thickness of about 0.020 inch.
Although the steel bar stock is a reasonably inexpensive starting material, the machining and chrome plating steps add to production and handling costs of manufacturing the rotor. In addition, during use, replacement costs of new rotors and the downtime costs when replacing worn rotors add to net cost of operating the pump.